The production of aluminum-lithium alloys has become of commercial interest, due to the combination of mechanical properties and light weight which these alloys exhibit. Unfortunately, molten aluminum-lithium alloys are very reactive with air which makes their production and fabrication correspondingly difficult.
The surface of an Al-Li bath reveals chemical behavior of molten lithium rather than aluminum thus causing the bath to: (1) burn on contact with air thus forming an excessive dross layer with the generation of toxic fumes resulting in poor lithium recovery and hazardous work conditions; (2) attract hydrogen from the atmosphere, including traces of water vapor, which increases hydrogen absorption and results in higher porosity levels and a loss of the desired mechanical properties; and (3) become practically unskimmable thus preventing proper stirring and degassing of the melt since any disruption of the generated dross will increase the rate at which further quantities of dross are formed. To overcome these enumerated difficulties, several solutions have been offered in the literature.
U.S. Pat. No. 4,248,630 discloses a process for adding alloying elements, including highly reactive metals such as lithium, to molten aluminum so that normally occurring oxidation reactions of such elements with the atmosphere is minimized. Basically, the process requires that all other alloying elements except lithium be added to the molten aluminum and the melt be degassed and filtered. Upon completion of the degassing/filtering step, the lithium is introduced into a mixing crucible as the final step prior to casting. The desired concentration of the lithium is achieved by controlling the relative amount of lithium and the alloyed melt. Uniformity of the mixture is achieved by mechanical stirring. The mixing crucible and all other crucibles in which lithium may be present are kept under an argon blanket.
U.S. Pat. No. 4,556,535 discloses a process for forming aluminum-lithium alloys which comprises continuously monitoring the ingot casting rate and continuously adding a measured and controlled amount of molten lithium beneath the surface of the molten aluminum stream as it flows to the ingot casting station. At the contact location of the lithium and aluminum, a mixture of argon and chlorine and/or other inert and reactive fluxing gases is injected through a vaned, rotating dispenser. The patent further discloses that the introduction of the lithium into the aluminum must be below the surface of the aluminum in order to minimize the occurrence of oxidation, fuming and hydrogen absorption.
Both U.S. Pat. Nos. 4,248,630 and 4,556,535 counterbalance the detrimental effects of lithium reactivity by means of minimizing time between the alloying and casting, however, neither process deals effectively with the problems of submerged injection of a premelted lithium charge, inert blanketing, lithium evaporation and melt hydrogen pick-up. Both systems suffer from the lack of proper melt surface protection for inert gas bubbling and handling operations.
Batch processes utilizing molten salt fluxes are an alternative to the continuous systems, discussed above, which are expensive and inflexible in operation, particularly when operating ranges or alloy changes are required. These fluxes, which are comprised primarily of lithium chloride or lithium fluoride, are applied to the surface of the lithium containing bath whereby they eliminate a part of the problem related to the lithium reactivity and still achieve a lithium recovery of approximately 80 wt %. Unfortunately, disruptions in the bath surface whether by stirring or degassing or any other movement in the bath breaks the flux layer and exposes the metal to ambient air resulting in violent oxidation of the lithium. Also, fluxes are highly corrosive to the refractory linings of the furnace and related casting equipment and materials of construction. The fluxes are also known to deteriorate the metal cleanliness and contaminate the environment as well as the equipment including melting, mixing, holding, and alloying furnaces, metal transfer troughs, casting stations, direct-chill liners and molds. Difficulties associated with storage and handling of the fluxes frequently cause a carry over of moisture into the aluminum-lithium melt and the subsequent oxidation and hydrogen pick-up.
Other solutions such as blanketing with a pure dry inert atmosphere eliminate the flux method drawbacks, however, these require tightly enclosed pots and troughs and therefore are not flexible enough to be used in various stages of aluminum-lithium fabrication. Furthermore, inert atmosphere blanketing does not decrease lithium evaporation from the bath, which results in substantial lithium losses and creates a potential hazard. Inert atmosphere blanketing does not provide flux layer cleaning properties such as preventing the hydrogen just removed from the bath during degassing from freely back-diffusing into the uncovered alloy, and/or allowing nonmetallic inclusions which have moved to the bath surface during inert gas stirring to be intercepted by the flux layer.